3D-printed guides have been used in endodontics to prepare conservative access, locate calcified or missing canals, and perform precisive osteotomy in apicoectomy. Here, we present treatment of a fusion tooth by combining 3D-printing technology and endodontic intervention in a 10-year-old patient.
The bifid crown of a maxillary right lateral incisor #7 had caused esthetic concerns and malocclusion. Clinical and radiographic examinations showed that #7 is fused with a supernumerary tooth with two independent root canals. The fusion involved the entire crown, coronal and middle roots. Due to financial constraints, a multidisciplinary approach involving endodontic, orthodontic and prosthodontic treatment was excluded. We hemisectioned the tooth intraorally with a 3D-printed guide, extracted the supernumerary tooth, and transplanted #7 to a position with improved esthetics and occlusion. A 3D-printed tooth replica was used to prepare the recipient site for autotransplantation. At 6-month follow-up, #7 was diagnosed with pulp necrosis and asymptomatic apical periodontitis. Root canal treatment of #7 was completed and osseous healing was observed 8 months later. Patient had no clinical symptoms and was satisfied with the outcome 14 months after hemisection and transplantation. The open space between #7 and #8 was closed without orthodontic treatment.
We present an alternative option to treat a fusion tooth in young patients who do not opt for other treatment options due to their stage of development or for socio-economic reasons. Techniques in modern endodontics, such as CBCT imaging and 3D-printing, should be adapted when it is beneficial to patients.
Anomalies in tooth morphology, which can occur either in primary or permanent dentition, are rare but can lead to significant clinical problems in esthetics, occlusion, and function. A bifid crown is usually mesio-distally larger than a normal tooth and may result from fusion, concrescence or gemination. Fusion is defined as the union of dentin and/or enamel between two or more tooth germs ( ) . Fusion can be the union of teeth within the same dentition or a normal tooth with a supernumerary tooth. It is more frequent in primary dentition (0.5 to 2.5 %) than the permanent dentition (0.1 to 1 %) , . Concrescence occurs when two teeth are joined by cementum. The prevalence is 0.04 % and occurs most commonly in upper second and third molars , . Tooth gemination is when two tooth crowns arise from one tooth germ ( ) . Gemination is also more prevalent in primary dentition (0.5 %) than in permanent dentition (0.1 %) ( ) . To differentiate between fusion and gemination, the bifid crown is counted as one. Fusion is indicated when the tooth number is one less than normal. A complete tooth number indicates gemination. However, when fusion occurs between a tooth within the dentition and a supernumerary tooth, the number of teeth remains normal. A fusion tooth usually has two separate root canal systems whereas a gemination tooth has only one root and one canal.
The etiology of fusion tooth remains unknown. The most common hypothesis is that physical force or pressure leads to direct contact of two tooth germs and causes necrosis of the epithelial tissue between two teeth during the developmental stage ( ) . Other possible causes include trauma, genetic and environmental factors , . The extent of fusion depends on the stage of tooth development at the time of union. Partial or incomplete fusion only involves the crown, while total or complete fusion involves fusion of both crowns and roots , . The root canal system of a fusion tooth can share a single pulp chamber dividing into two root canals or have two independent root canal systems , .
Intervention of tooth fusion is needed when the tooth presents a pulpal/periapical/ periodontal infection or when esthetics and function are compromised. Several treatment options for fusion have been reported including extraction, root canal treatment, hemisection, and a multidisciplinary approach involving endodontic, orthodontic and prosthodontic treatment ( ) . To achieve appropriate treatment planning, it is essential to thoroughly analyze the root and canal anatomy of a fusion tooth. CBCT imaging offers images in all spatial planes and reconstructive volumetric images. It provides detailed information of root canal configurations, which may not be revealed by traditional radiographs ( ) . In addition, CBCT images merged with virtual images of teeth can be used to design a 3D-printed guide for non-surgical as well as surgical procedures , . In this case report, we describe successful treatment of a fusion tooth by hemisection and autotransplantation with the aid of a 3D-printed guide and a tooth replica.
Clinical Examination and Treatment Planning
A 10-year-old female patient with a bifid crown of #7 was referred to the Endodontic Clinic at University of Connecticut Health for evaluation and treatment. Medical history was unremarkable. Patient and her mother expressed that their main concern was esthetics and limited finances for dental treatment. The referring pediatric dentist further stated the concern of malocclusion caused by the heavy contact between the fusion tooth and mandibular right lateral incisor #26. Patient had no past history of pain. An extraoral examination showed normal tissues and no lymphadenopathy. The intraoral examination revealed a bifid crown of #7 with no caries or restoration ( Figure 1 A and B). Both crowns of the fusion tooth responded normally to cold (Endo Ice, Coltene, Alstätten, Switzerland) and EPT tests (SybronEndo Vitality Scanner, Kerr, Orange, CA). The fusion tooth had no mobility and all probing depths were less than 3 mm. A periapical radiograph showed a two-roots fused tooth with no apparent periapical pathology ( Figure 1 C). The CBCT image showed that the fusion involved the entire crown, coronal and middle roots ( Figure 1 D). Both roots of the fusion tooth had immature apices ( Figure 1 E). Based on the clinical and radiographic examinations, a fusion tooth of #7 and a supernumerary tooth with normal pulp and periapical status was diagnosed. To treat this fusion tooth with the least expense, we proposed to hemisection the fusion tooth, discarded the buccally-located tooth, and transplant the palatally-located #7 to achieve better esthetics and occlusion. We discussed the treatment plan and the guarded prognosis with the patient and her parents. We chose to discard the buccally-located tooth because the coronal pulp obliteration would make future root canal treatment more challenging ( Figure 1 F). The patient and her parents were advised that root canal treatment will be needed when the transplanted tooth becomes non-vital or symptomatic at follow-up appointments.
To avoid the tooth fracture during extraction and to minimize the extraoral operation time, we decided to perform intraoral hemisection. The CBCT image revealed that the crown of unerupted canine #6 was in close proximity to the cutting path of hemisection. We decided to fabricate a 3D-printed guide to separate two roots in a safe and controllable manner.
Fabrication of the 3D-Printed Guide
The procedures are described in 3 steps:
Image processing ( Figure 2 A and B): We took a digital impression of the upper arch with an intraoral scanner CEREC Omnicam (Dentsply Sirona, Bensheim, Germany). CBCT and intraoral images were processed as previously described ( ) . Briefly, CBCT images were segmented in an editor for DICOM files and all images were converted to STL files. An additional STL file of the palatally-located #7 was created for printing the tooth replica. These STL files were merged in a 3D triangular meshes editor software (Meshlab, ISTI, Pisa, Italy). After the merging process, the STL files were introduced into another 3D mesh editor software (Meshmixer, Autodesk Inc., Ontario, Canada).
Design ( Figure 2 C and D): Design of the 3D-printed guide was based on the information revealed from the CBCT images, including the involvement of fusion, angulation and length of roots, the distance to adjacent teeth and the thickness of cortical bone. The cutting path was determined to be 1.9 mm away from tooth #6. The cutting window of the guide had a tight and precise fit of the shank of the high-speed bur. The bucco-lingual width of the window was designed to be 6.5 mm to achieve the complete separation of two roots and to preserve the buccal and lingual cortical bone. The 3D-printed guide spanned from tooth #A to #9 to ensure stability. Four openings were created near the cutting window to allow sufficient cooling with normal saline irrigation.
3D-printing ( Figure 2 E and F): The guide and a tooth replica of #7 were printed in a 3D Printer (Formlabs 2, Formlabs, Somerville, MA) using FDA approved Dental SG Resin (Formlabs, Somerville, MA).